It has long been common practice to cast tubular metal articles centrifugally, using a permanent mold which has an active mold surface of circular transverse cross-section, the mold being rotated about the longitudinal axis of the active mold surface. Centrifugal casting molds are made of metal which has a melting point which may not be markedly different from that of the metal being cast, and it is therefore necessary to cover the active mold surface with a lining of a material which will protect the mold from damage by contact with the molten casting metal, prevent the casting from picking up material from the mold surface, and allow the finished casting to be separated from the mold. One method employed by prior art workers for lining centrifugal casting molds has been to apply to the active mold surface a slurry of a finely particulate refractory material, typically zircon powder or silica powder, that method having been used for stationary, non-centrifugal molds as disclosed in U.S. Pat. No. 1,662,354 to Harry M. Williams, and adopted for centrifugal casting, as described in U.S. Pat. No. 3,527,285 to Fred J. Webbere. While they have achieved considerable acceptance, such practices have presented substantial disadvantages, particularly because of the need for venting to dispose of water vapor generated during casting, and because the coatings provided on the mold surface have not always been adequately strong and uniform and have tended to be penetrated by the molten metal being cast, with resulting roughness of the cast surface and increased machining difficulties due to presence of refractory particles in the cast metal. In efforts to avoid such deficiencies, it has been proposed to employ resin binders and other non-inert ingredients as shown for example in U.S. Pat. No. 3,056,692 to Koshiro Kitada, but such coatings are unduly expensive and tend to generate gaseous products at casting temperatures so that the mold must be vented. As disclosed for example in U.S. Pat. No. 3,110,067 to Donald C. Abbott, it has been proposed to spray a resin binder onto the surface of a heated relatively thick pre-formed refractory layer with the intent of eliminating the need for venting the mold but, at best, that practice still requires the use of both a relatively expensive refractory material and a relatively expensive resin.
It has also been proposed to apply only the particulate refractory material, without water or other liquid carrier material and without additive binders such as bentonite or resin, primarily to control the grain structure of the cast metal. As disclosed in U.S. Pat. No. 1,949,433 to Norman F. S. Russell et al, such methods employ a carrier gas to carry the particulate refractory material onto the active mold surface immediately in advance of the casting metal and depend upon centrifugal force to establish a very thin coating layer of the refractory material, said to be limited to not more than 0.025 mm. in thickness. Such methods have been adopted for casting some articles, such as pipes, which do not require a particularly smooth outer surface, but are not suitable for products, such as engine cylinder liners, which require a relatively smooth outer surface free of chilled iron. The as-cast surface is usually quite rough, so that substantial machining would be required for finished castings with a smooth outer surface, and the nature of the thin coating of particulate refractory material has been such that particles of the refractory material are picked up by the cast article and interfere seriously with machining by slowing the machining rate and drastically reducing cutting tool life. Use of a thin coating of refractory material also limits the practice to production of articles which have no outer enlargements unless, as in the case of a pipe with an end bell, the enlargement can be outwardly tapering and located at the very end of the mold. Further, such very thin linings do not provide thermal insulation adequate to delay the solidification of the molten iron, when iron is the metal being cast, sufficiently to cause Type A graphite to be formed, a definite requirement for cast articles such as cylinder liners and bearings.
A further disadvantage of prior art methods arises from the relative cost of the refractory material and the difficulty in recovering that material, after casting, for reuse. Materials such as zircon flour have a per pound cost greater than that of the metal being cast. When additive materials such as clays, bentonite or resins are employed, recycle of the refractory material is impractical. When only a thin layer, such as that disclosed in U.S. Pat. No. 1,949,433, is employed, much of the refractory material is simply lost, by being picked up by the casting and otherwise, so that recovery is at best difficult and costly.